As the ozone deletion causes more UV radiation to reach the earth's surface, the effect of UV light included in the sunlight on a human body and environment is becoming a greater concern.
The three types of UV radiation are classified as follows: long-wavelength ultraviolet light (UV-A: wavelength approximately 320 nm to 400 nm); medium-wavelength ultraviolet light (UV-B: wavelength approximately 280 nm to 320 nm); and short-wavelength ultraviolet light (UV-C: wavelength approximately 280 nm or shorter), and the effect on a human body and environment differs depending on the wavelength. UV-A can penetrate into the deeper layers of the skin and is responsible for the immediate tanning effect. It also contributes to skin aging. UV-B is responsible for skin burning, and promotes the development of skin cancer. UV-C has a strong bactericidal effect, but it is almost completely filtered by the ozone layer and little reaches the earth's surface.
It has been significantly important to immediately detect the amount of daily UV radiation in order to protect a human body, and the UV index, which functions as an indicator of the UV radiation amount, was introduced in 1995. The UV index indicates a relative degree of impact on a human body, and can be calculated using the CIE action spectrum, which was defined by CIE (Commission Internationale de l'Eclairage).
Against this background, there is a strong demand for a technique to accurately detect the radiation amount of UV-A and UV-B, respectively, included in the ultraviolet light. There is also a stronger demand for a technique to measure the UV radiation amount that can detect the radiation amount of UV-B and the total amount of UV-A and UV-B at the same time.
Japanese Patent Application Laid-open Publication No. 2008-251709 (Patent Document 1) discloses a UV light receiving element in which the first filter that allows UV-A, UV-B and visible light through is disposed on the first photo-diode, and the second filter that allows UV-A and visible light through is disposed on the second photo-diode. With this configuration, it is possible to separately detect the radiation amount of UV-A and the radiation amount of UV-B based on the radiation amount detected by the first photo-diode and the radiation amount detected by the second photo-diode. In the UV light receiving element disclosed in Patent Document 1, a difference in the light absorbing property between the first filter and second filter is determined by the amount of hydrogen contained in a silicon nitrogen film included in each filter.
WO 2012/137539 Pamphlet (Patent Document 2) discloses a UV light sensor in which a protective film and a filter film are stacked on a photo-diode. The protective film has a high light transmittance in the wavelength range of UV-A and UV-B, and the filter film is made of a multi-layer film formed by alternately stacking a low refractive index material and a high refractive index material and having a high light transmittance in the wavelength range of UV-A and UV-B, the filter film also having the filter property that can block UV-C and visible light.